working my way through structure and interpretation of computer programs, ch 3

sicp_ch_3.rkt

#lang sicp

(define balance 100)

(define (withdraw amount)
  (if (>= balance amount)
      (begin (set! balance (- balance amount))
             balance)
      "Insufficient funds"))


(define new-withdraw
  (let ((balance 100))
    (lambda (amount)
      (if (>= balance amount)
          (begin (set! balance (- balance amount))
                 balance)
          "Insufficient funds"))))

(define (make-withdraw balance)
  (lambda (amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds")))


(define (make-account balance)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (dispatch m)
    (cond ((eq? m 'withdraw) withdraw)
          ((eq? m 'deposit) deposit)
          (else (error "Unknown request -- MAKE-ACCOUNT"
                       m))))
  dispatch)

; exercise 3.1
(define (make-accumulator acc)
  (define (accumulate amt)
    (set! acc (+ acc amt))
     acc)
  accumulate)

; exercise 3.2
(define (make-monitored f)
  (let ((n 0))
    (lambda (arg)
      (if (eq? arg 'how-many-calls?)
          n
          (begin
            (set! n (+ n 1))
            (f arg))))))

; exercise 3.3
(define (make-account2 balance password)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (dispatch pw m)
    (if (eq? pw password)
        (cond ((eq? m 'withdraw) withdraw)
              ((eq? m 'deposit) deposit)
              (else (error "Unknown request -- MAKE-ACCOUNT"
                           m)))
        (error "Incorrect password")))
  dispatch)

; exercise 3.4
(define (make-account3 balance password)
  (define consecutive-wrong-pw 0)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (dispatch pw m)
    (if (eq? pw password)
        (begin
          (set! consecutive-wrong-pw 0)
          (cond ((eq? m 'withdraw) withdraw)
                ((eq? m 'deposit) deposit)
                (else (error "Unknown request -- MAKE-ACCOUNT"
                             m))))
        (begin
          (set! consecutive-wrong-pw (+ consecutive-wrong-pw 1))
          (if (eq? consecutive-wrong-pw 7)
              (error "call-the-cops")
              (error "Incorrect password")))))
  dispatch)


(define (rand-update x) 
         (remainder (+ (* 13 x) 5) 24)) 
(define random-init (rand-update (expt 2 32))) 

(define rand
  (let ((x random-init))
    (lambda ()
      (set! x (rand-update x))
      x)))


(define (estimate-pi trials)
  (sqrt (/ 6 (monte-carlo trials cesaro-test))))
(define (cesaro-test)
   (= (gcd (rand) (rand)) 1))
(define (monte-carlo trials experiment)
  (define (iter trials-remaining trials-passed)
    (cond ((= trials-remaining 0)
           (/ trials-passed trials))
          ((experiment)
           (iter (- trials-remaining 1) (+ trials-passed 1)))
          (else
           (iter (- trials-remaining 1) trials-passed))))
  (iter trials 0))


(define (random-in-range low high)
  (let ((range (- high low)))
    (+ low (random range))))

; exercise 3.5
(define (estimate-integral P x1 x2 y1 y2 trials)
  (define (iter trials-remaining trials-passed)
    (if (< trials-remaining 0)
        (/ trials-passed trials)
        (iter ;not a tail-call, but more readable this way
         (- trials-remaining 1)
         (+ trials-passed
            (if (P (random-in-range x1 x2) (random-in-range y1 y2))
                1
                0))))
    )
  (iter trials 0))

; measure area of unit circle with estimate-integral
(define (sum vs) (if (eq? vs nil) 0 (+ (car vs) (sum (cdr vs)))))
(define (sum2 vs) (apply + vs))
(define (vector-len vs)
  (define (sq x) (* x x))
  (sum (map sq vs)))
(define unit-circle-area
  (estimate-integral
   (lambda (x y) (< (vector-len (list x y)) 1))
   -1 1 -1 1 10000))

; exercise 3.6
(define rand2
  (let ((x random-init))
    (lambda (cmd)
      (cond
         ((eq? cmd `generate)
          (set! x (rand-update x))
          x)
         ((eq? cmd `reset)
          (lambda (x_) (set! x x_)))))))